Let's face it, you create a new document,copy and paste a new image onto the canvas, and try to position it to the center, if you're a beginner, you may try to judge the center point by using your eyes. But believe me, your eyes don't always tell the truth.
That's when the alignment options come in handy. Simply click on the “Move”tool ( shown below).
Then you will see the alignment options appear on the top (menu bar) that I have hi-lighted and shown below.
And let's say you want to position an object in the center of the canvas, simply Ctrl+A to select the whole canvas,then you will see some alignment options become clickable.
And then you simply click on "Align Vertical Centers" and "Align Horizontal Centers" you will see the object is now positioned in the center of the canvas.
Like any thing else, the more often you do this, the quicker it becomes second nature.